The invention discloses a methodology for detecting magnetic beads, magnetic particles and magnetic nano-particles by inducing ferromagnetic resonance (FMR) of their magnetic moment and by using a magneto-resistive (MR) sensor to detect the magnetic field produced by the FMR. Binding these magnetic beads or particles to biological or chemical molecules thus enables the presence of these molecules to be detected. In the form of a binding assay, where a matrix of MR sensors are patterned, this method can be used to identify the presence of a molecule of interest as well as to quantify its population. This method can address many other issues in the prior art that also utilize magnetic bead labeling and MR sensing, thus making this technology widely applicable.
Detection of biological or chemical molecules by using super-paramagnetic beads and particles as the labeling component and by using magneto-resistive sensors for detection of such labels is regarded as a promising technique to achieve accurate molecule counting with a resolution of several molecules or even a single molecule. It has the potential to enable fast, efficient, and economical biological applications, such as in-field virus and bacteria detection.
Binding assays to detect target molecules is a widely used technique in the biological, biochemical, and medical communities. The selective bindings commonly used include polynucleic acid bindings or hybridizations involving RNA and DNA, many types of ligand-to-receptor bindings, as well as antibody to antigen bindings. The target molecules in these bindings, for example, proteins or DNA, can also be a distinctive component or product of viruses, bacteria and cells, which may be the actual objects of interest for the detection.
In a binding assay, the binding molecules are attached to a solid substrate as “capture molecules”. When the assay is exposed to a liquid-form sample, where the target molecules attached to a physical label are contained, the binding molecules capture the target molecules with the selective bindings and immobilize the target molecules on the surface. This capture process is called “recognition”. The recognition events can be made to generate detectable signals from the attached labels and, consequently, the presence or absence of a target molecule can be detected.
In various prior art techniques utilizing the labeled binding process, the labels originally attached to the target molecule are also immobilized on the surface after the recognition process. The labels are either bound together with the target molecules on the surface (“sandwich” assay) or are by themselves (“competitive” assay). After the removal of non-bound labels, the bound labels can then be made to generate measurable signals.
When using magnetic beads or particles as the labeling component and a MR sensor as the detector, the MR devices are embedded below the binding surface and are usually covered by a protective layer. When the magnetic beads or particles are bound to the surface above a MR sensor, they can generate a magnetic field spontaneously or, in the case of super-paramagnetic beads or particles, if an applied magnetic field is present. This magnetic field from the beads or particles can be sensed by the MR device, which can then provide a voltage signal.
The magnetic labels described in published studies or as patents are usually super-paramagnetic beads—nano-particles (or larger beads that comprise nano-particles suspended in a non-magnertic matrix) that have no magnetization at room temperature without an externally applied magnetic field because of the super-paramagnetic effect. Such labels are desired in biological applications because they do not aggregate (at zero field). The beads or particles described in the prior art usually range in size from tens of nanometers to several microns. When the labels are attached to a surface after the recognition process, either single or multiple labels are attached to each MR device. However, the sensing mechanism has generally been the same for all the previous designs. When the magnetic labels are attached to the MR sensor top surface, the field generated by the magnetic moment of the beads or particles will either act directly on the underlying MR sensor or it may cancel a portion of the applied magnetic field acting on the sensor. For sensors that have no attached labels, the magnetic field from the magnetic labels is not present.
FIG. 1 is a schematic representation of the scheme outlined above. Magnetic bead 1 (which term will, hereinafter, be assumed to include magnetic beads, as well as particles and nano-particles) is attached to the MR sensor surface by the binding pairs 5 after recognition. The MR sensor used or referred is usually a giant-magneto-resistive (GMR) or a tunneling-magneto-resistive (TMR) sensor, which contains a magnetic free layer 2, a non-magnetic spacer layer 3 and a magnetic reference layer 4. Spacer 3 is usually a conductive layer for GMR sensors and insulator for TMR sensors. Magnetization of reference layer 4, as represented by Mreference, reference, is fixed (i.e. ‘pinned’) in the X axis direction by an exchange field from other magnetic layers below it, which are not shown in the figure.
The pinned layer does not change its magnetization direction under normal magnetic fields. In a conventional MR sensor, a bias DC field Hbias is usually applied in the Y direction by a pair of opposing hard magnets on the sides of the sensor, so that the free layer's magnetization will be in the Y axis direction when no magnetic field is applied. However, this free layer alignment to Y axis can also be achieved by making the sensor dimension in the Y axis longer than in X axis due to the shape anisotropy. Because of the shape anisotropy, the magnetization of the free layer 2, as represented by Mfree, can only rotate freely in the XY plane when a transverse field is applied along the X axis direction and it is very difficult to rotate outside the XY plane, i.e. towards the Z axis.
If a magnetic field is applied in the X axis direction, the free layer magnetization rotates away from the Y axis and the resistance of the entire MR junction will change according toR=R0−ΔR cos θ
where R0 is the base resistance of the sensor, ΔR is the full range resistance change of the sensor and θ is the angle between the magnetization of the reference layer and the free layer. With a DC current applied to the device, where the current can either flow in the XY plane or perpendicularly through the device, the voltage across the device will change, because of the resistance change, to produce a measurable voltage signal.
In prior studies and patents, several detection schemes were used. One commonly used scheme is to apply a magnetic field in the transverse direction [4-10, 12-13], e.g. along the X axis as in FIG. 1. Super-paramagnetic beads are also used. When a super-paramagnetic bead is bound to the top surface of the MR sensor, this applied field can magnetize the bead magnetization along the field direction. The bead magnetic moment in turn will produce a magnetic field in the MR sensor below and partially cancel the original applied magnetic field acting on the MR ‘sensor. Therefore, the voltage across the sensor when a bead is present is different than when there is no bead attached at the same applied field condition and the presence of the bead is detected by this voltage amplitude difference.
It is important to note that these prior art methods use applied magnetic fields oscillating at frequencies less than 100 kHz (often much less) whereas the present invention requires frequencies in the MHZ (and higher) region.
A reference sensor to which beads will not attach to at any time is normally used for comparison of this voltage difference. During the detection, the applied field can also be a modulated by an AC field that will induce a same frequency AC voltage across the sensor. By utilizing a lock-in technique, the signal to noise ratio can be enhanced.
Another bead sensing scheme, also known as BARC [1-3, 11], is to apply a DC field perpendicular to the film plane, i.e. along the Z axis direction in FIG. 1, with no bias field at all being present. This DC field serves to magnetize the super-paramagnetic bead moment vertically. The in plane component of the field generated by the vertical bead moment will rotate the free layer magnetizations in the upper XY plane and low XY plane towards or away from the Y axis at the same time. If the reference layer magnetization is aligned along the Y axis, or a multi-layer MR structure is used, this rotation will produce a resistance change. It is also called “scissoring mode” [11]. This scheme also needs a reference sensor for detection.
A potential problem common to all the previously published or patented bead-MR sensor detection methods is that they are prone to fluctuations in the magnetic signal. Since all these detection methods are mainly practiced in the low frequency region from several Hertz to several kHertz, 1/f noise is very significant. Although the lock-in technique can successfully suppress the noise from the electrical sources by its narrow bandwidth, the noise from magnetic sources, for example Barkhausen noise, popcorn noise and telegraph noise cannot be prevented from affecting the locked-in signal. These magnetic noise sources are related to domain and local magnetization switching of MR sensors and are always most predominant in the low frequency region and usually show up as signal level random fluctuations. For the scheme that requires a reference sensor for detection, this combined noise effect from both the detection sensor and the reference sensor will at least double this parasitic fluctuation of the signal level.
Besides the fluctuations from the sensor, the bead itself will always have shape, size and magnetic content variations as well as binding site variations. These variations will also cause fluctuations of the amount of the bead magnetic field going into the sensor. With all the noise sources added together, these can be quite large and can cause significant signal fluctuations to inhibit practical binding assay applications that are based on the detection of the absolute field strength.
Another problem specifically for the field cancellation method is that a magnetic field needs to be applied in the sensing direction, i.e. X axis, to magnetize the magnetic beads and thus generate the cancellation field. However, this relatively large amplitude field will rotate the MR sensor free layer magnetization to the place where its sensitivity is not the highest. In other words, when the bead field is highest, the sensor sensitivity is lowest. By proper design of the MR sensor film structure and by using a vertical AC field to mimic the scissoring mode, second harmonic detection [7] can minimize the sensitivity loss. But the signal generated by the cancellation effect from the bead field is significantly decreased because it is only operating with a single bead field polarity and not a full reversal of the bead field direction in the transverse direction.
For the scissoring mode, where the bead is magnetized vertically, there is no sensitivity concern. However, this mode requires a relatively large sensor size. For a sub-micron or deep submicron size sensor, the exchange energy within the free layer will degrade the amount of rotation achievable for the two regions of the sensor rotating against each other. A low bias field, Hbias, or no bias field may be needed, which can easily lead to instability in a micron size sensor because of weak or no free layer domain control. In addition, for a GMR or TMR sensor with a single free layer, since this scheme only utilizes the sensor free layer magnetization rotation between 0 and 90 degrees, half of the sensor sensitivity region is not used.
For a multilayer GMR sensor, the rotation of magnetization can theoretically reach maximum or 0 to 180 degrees. A current-in-plane (CIP) multilayer GMR sensor usually has a lower dR/R, i.e. lower signal, than the state-of-the-art TMR or carefully designed spin valve GMR sensors. The current-perpendicular-to-plane (CPP) multilayer GMR sensors although possessing a much higher dR/R than the CIP ones, have also shown extraordinary magnetically related 1/f type noise in previous studies.
This noise can be more than 10 dB over the sensor's Johnson noise level in a micron size multilayer device and it will severely degrade the SNR of the sensor. To overcome the above problems, what is needed is, first, a scheme that can avoid having the bead magnetizing field affect the sensor free layer as well as utilizing the full reversal of the bead magnetization to gain maximum signal. Second, to avoid fluctuation of the bead magnetic field or the sensor resistance due to various magnetic sources, the method should not rely on detection of the absolute bead field magnitude. Third, signal detection at much higher frequencies than currently being explored (<100 kHz), for example beyond 1 MHz, is preferred in order to reduce low frequency noise effects.
A routine search of the prior art was performed with the following references of interest being found:    [1] D. R. Baselt et al., “A biosensor based on magnetoresistance technology,” Biosens. Bioelectron., vol. 13, pp. 731-739, October 1998.    [2] R. L. Edelstein et al., “The BARC biosensor applied to the detection of biological warfare agents,” Biosens. Bioelectron., vol. 14, pp. 805-813, January 2000.    [3] M. M. Miller et al., “A DNA array sensor utilizing magnetic microbeads and magnetoelectronic detection,” J. Magn. Magn. Mater., vol. 225, pp. 138-144, April 2001.    [4] D. L. Graham, H. Ferreira, J. Bernardo, P. P. Freitas, and J. M. S. Cabral, “Single magnetic microsphere placement and detection on-chip using current line designs with integrated spin valve sensors: Biotechnological applications,” J. Appl. Phys., vol. 91, pp. 7786-7788, May 2002.    [5] H. Ferreira, D. L. Graham, P. P. Freitas, and J. M. S. Cabral, “Biodetection using magnetically labeled biomolecules and arrays,” J. Appl. Phys., vol. 93, pp. 7281, May 2003.    [6] G. Li et al., “Detection of single micron-sized magnetic bead and magnetic nanoparticles using spin valve sensors for biological applications,” J. Appl. Phys., vol. 93, pp. 7557-7559, May 2003.    [7] G. Li, S. X. Wang and S. Sun, “Model and experiment of detecting multiple magnetic nanoparticles as biomolecular labels by spin valve sesnors,” IEEE Trans. Magn., vol. 40, pp. 3000, 2004    [8] S. X. Wang et al., “Towards a magnetic microarray for sensitive diagnostics,” J. Magn. Magn. Mater., vol. 293, pp. 731-736, 2005.    [9] W. Shen, X. Liu, D. Mazumdar and G. Xiao, “In situ detection of single micron-sized magnetic beads using magnetic,” Appl. Phys. Lett., vol. 86, pp. 253901, 2005.    [10] H. Ferreira, N. Feliciano, D. L. Graham and P. P. Freitas, “Effect of spin-valve sensor magnetostatic fields on nanobead detection,” J. Appl. Phys., vol. 97, pp. 10Q904, 2005.    [11] D. R. Baselt, “Biosensor using magnetically detected label,” U.S. Pat. No. 5,981,297 (1999) teaches that a change in output of MR sensors indicates the presence of magnetic particles.    [12] M. C. Tondra, “Magnetizable Bead Detector,” U.S. Pat. No. 6,743,639 B1 (2004)    [13] M. C. Tondra, “Magnetizable Bead Detector,” U.S. Pat. No. 6,875,621 B2 (2005); this, and ref. 12 above, shows an MR sensor in a bridge circuit which may comprise interconnected individual sensors adjacent to the binding molecule layer.    [14] U.S. Pat. No. 6,518,747 (Sager et al) discloses applying an AC signal to excite Hall sensors in a DC field to detect magnetic particles.    [15] U.S. Patent Application 2006/0020371 (Ham et al) discusses FMR detection of magnetic beads.